1. Field of the Invention
The present invention relates to immersion lithography tools, and more particularly, to an immersion lithography tool with an element, positioned between an immersion element and an object (e.g., a substrate), to prevent or reduce leakage of immersion fluid when the object is moved.
2. Related Art
A typical lithography tool includes a radiation source, a projection optical system, and a substrate stage to support and move a substrate to be imaged. A radiation-sensitive material, such as resist, is coated onto the substrate surface prior to placement onto the substrate stage. During operation, radiation energy from the radiation source is used to project an image defined by an imaging element through the projection optical system onto the substrate. The projection optical system typically includes a number of lenses. The lens or optical element closest to the substrate is often referred to as the “last” or “final” optical element.
The projection area during an exposure is typically much smaller than the imaging surface of the substrate. The substrate therefore has to be moved relative to the projection optical system to pattern the entire surface. In the semiconductor industry, two types of lithography tools are commonly used. With so-called “step and repeat” tools, the entire image pattern is projected at once in a single exposure onto a target area of the substrate. After the exposure, the wafer is moved or “stepped” in the X and/or Y direction and a new target area is exposed. This step and repeat process is performed over and over until the entire substrate surface is exposed. With scanning type lithography tools, the target area is exposed in a continuous or “scanning” motion. The imaging element is moved in one direction, while the substrate is moved in either the same or the opposite direction during exposure. After each scan, the substrate is then moved in the X and/or Y direction to the next scan target area. This process is repeated until all the desired areas on the substrate have been exposed.
It should be noted that lithography tools are typically used to image or pattern semiconductor wafers and flat panel displays. The term “substrate”, as used herein, is intended to generically mean any work piece that can be patterned, including, but not limited to, semiconductor wafers and flat panel displays.
Immersion lithography systems use a layer of fluid that fills a gap between the final optical element of the projection optical system and the substrate. The fluid enhances the resolution of the system by enabling exposures with a numerical aperture (NA) greater than one, which is the theoretical limit for conventional “dry” lithography. The fluid in the gap permits the exposure with radiation that would otherwise be completely internally reflected at the optical-air interface. With immersion lithography, numerical apertures as high as the index of refraction of the fluid are possible. Immersion also increases the depth of focus for a given NA, which is the tolerable error in the vertical position of the substrate, compared to a conventional dry lithography system. Immersion lithography therefore has the ability to provide greater resolution than can be performed using conventional dry lithography, as the fluid essentially becomes part of the optical system of the lithography tool.
One known way of maintaining the immersion fluid in the gap where exposure of the substrate is to occur is with the use of an air curtain. For more information on air curtain type immersion tools, see for example U.S. Patent publication 2005/0007569 or European Patent Applications EP 1 477 856 A1 and EP 1 420 299 A2, incorporated by reference herein for all purposes.
It is also known to maintain the immersion fluid in the gap between the last optical element and the imaging surface of the substrate by submersing both in a container filled with immersion fluid. See for example U.S. Pat. No. 4,509,852, also incorporated by reference herein.
Another known way of maintaining the immersion fluid within the gap of a lithography tool is with the use of a confinement member that surrounds the last optical element immediately above the area to be exposed on the substrate. For more information on confinement member type immersion lithography tools, see U.S. application Ser. No. 11/362,833, and PCT Application Serial Numbers. PCT/US2004/22915 and PCT/US2005/14200, all incorporated herein by reference for all purposes, all incorporated herein by reference for all purposes.
In yet another approach, which is a variation of the above-described submersion type tool, a large confinement plate is used for submerging the substrate to be imaged in the immersion fluid. For more details on confinement plate type immersion lithography tools, see U.S. patent publication 2007/0279608, incorporated by reference herein.
During semiconductor wafer fabrication for example, wafers are typically patterned one after another by the lithography tool. After a wafer has been patterned, it is replaced and the next wafer is exposed. This process is completed over and over, typically as fast as possible, to increase throughput. During a wafer exchange, the just exposed wafer typically has to be moved a relatively long distance from the exposure area to the wafer exchange area. Once the exchange takes place, the new wafer undergoes another relatively long-move to an alignment area. After alignment, the wafer undergoes yet another long-move back to the exposure area for exposure. For the sake of simplicity, all of the above-described moves are hereafter generically referred to as “long-moves”.
Long-moves can be problematic with confinement member type immersion tools under certain circumstances. If the speed during a long-move is too fast, there is a tendency for the immersion fluid to leak out from under the confinement member, leaving a trail of water behind on the wafer. This problem can be either mitigated or altogether eliminated by reducing the speed of the long-moves. The drawback of the reduced speed, however, is that throughput is reduced as well.